Steer-by-wire steering apparatus

This application claims priority to Japanese Patent Application No. 2023-28280 filed on Feb. 27, 2023, which is incorporated herein by reference in its entirety including the specification, claims, drawings, and abstract.

The present disclosure relates to a steer-by-wire steering apparatus, and in particular to a steering reaction force applied to a steering wheel.

In conventional steering apparatuses, a steering wheel manipulated by a driver is mechanically linked to a vehicle wheel which is to be turned. This contrasts with steer-by-wire steering apparatuses, in which the steering wheel is not mechanically linked to the vehicle wheel to be turned. In a steer-by-wire steering apparatus, a steering angle which is an angle of steering manipulation of the steering wheel is converted into an electrical signal, and a turning actuator turns the vehicle wheel based on this electrical signal. Generally, the ratio of the turning angle with respect to the steering angle is larger in a steer-by-wire steering apparatus in comparison to the conventional steering apparatuses. That is, when the steering angle is the same, the turning angle of the turning wheel is larger in the steer-by-wire steering apparatus than in the conventional steering apparatus.

In addition, a reaction force actuator applies a steering reaction force opposing the steering of the driver is applied to the steering wheel. This makes it possible for the driver to understand road surface situations and limits of the steering range through the steering reaction force.

JP 2021-115937 A discloses a technique in which the steering reaction force is increased according to an increase of the steering angle in a region where a cornering force characteristic becomes nonlinear. The nonlinear region described above is defined based on a lateral acceleration of a vehicle and a torque for turning the turning wheel. When a slip angle of the turning wheel becomes large, the cornering force characteristic becomes nonlinear. The increase in the steering reaction force suppresses excessive manipulation of the steering wheel.

In the region where the cornering force characteristic becomes nonlinear, the increase in a cornering force is small in comparison to the increases in the steering angle and the turning angle. Therefore, steering and turning of the wheel in this region is not effective in relation to the increase in the cornering force.

On the other hand, when the driver suddenly rapidly steers the steering wheel, the steering angle to be manipulated by the driver does not significantly differ between the steer-by-wire steering apparatus and the conventional steering apparatus. In the meantime, as described above, in the steer-by-wire steering apparatus, because of the large ratio of the turning angle with respect to the steering angle, the turning angle becomes large. When the turning angle becomes large at a time of rapid steering, there may be cases in which the cornering force characteristic becomes nonlinear. As described above, the turning of the vehicle wheel in the nonlinear region is not effective, and excessive turning results. It is difficult to suppress the excessive turning if the steering reaction force is increased after the cornering characteristic became nonlinear.

An advantage of the present disclosure lies in suppression of excessive turning of the wheel in relation to rapid steering.

According to one aspect of the present disclosure, there is provided a steer-by-wire steering apparatus comprising a steering angle sensor that detects a steering angle of a steering wheel; a turning actuator that turns a turning wheel according to a target turning angle which is a control target value of a turning angle of the turning wheel, calculated based on the steering angle; a turning angle sensor that detects an actual turning angle which is an actual turning angle of the turning wheel turned by the turning actuator; and a reaction force actuator that applies a steering reaction force, opposing a steering manipulation of a driver, to the steering wheel. The reaction force actuator applies, when the steering angle becomes close to an end of a virtual steering range which is a virtual steering range of the steering wheel, an end steering reaction force, which rapidly increases with the approaching to the end of the virtual steering wheel, to the steering wheel. Further, the reaction force actuator applies the end steering reaction force to the steering wheel based on the virtual steering range which is narrower than that during a normal time, during rapid steering in which a turning angle velocity calculated based on the actual turning angle is greater than or equal to a predetermined value, or a steering angle velocity calculated based on the steering angle is greater than or equal to a predetermined value.

By narrowing the virtual steering range during the rapid steering, excessive turning of the wheel can be suppressed.

During rapid steering, when a turning follow delay which is a difference between the target turning angle and the actual turning angle is greater than or equal to a predetermined value, the reaction force actuator may apply the end steering reaction force to the steering wheel based on the virtual steering range of the normal time in place of the narrower virtual steering range. It is thereby possible to suppress narrowing of the steering range when a counter-steer manipulation is to be performed.

The reaction actuator may apply the end steering reaction force to the steering wheel based on the virtual steering range which becomes narrower as the turning angle velocity becomes larger, and/or as the turning follow delay becomes larger.

According to another aspect of the present disclosure, there is provided a steer-by-wire steering apparatus comprising a steering angle sensor that detects a steering angle of a steering wheel; a turning actuator that turns a turning wheel according to a target turning angle which is a control target value of a turning angle of the turning wheel, calculated based on the steering angle; and a reaction force actuator that applies a steering reaction force, opposing a steering manipulation of a driver, to the steering wheel. The reaction force actuator applies, when the steering angle becomes close to an end of a virtual steering range which is a virtual steering range of the steering wheel, an end steering reaction force, which rapidly increases with the approaching to the end of the virtual steering range, to the steering wheel, and applies the end steering reaction force to the steering wheel based on the virtual steering range which is narrower than that during a normal time, during rapid steering in which a steering angle velocity calculated based on the steering angle is greater than or equal to a predetermined value.

An embodiment of the present disclosure will now be described with reference to the drawings.is a diagram schematically showing a structure of a steering apparatus. The steering apparatusis a steer-by-wire steering apparatus. In the steer-by-wire steering apparatus, a steering wheel manipulated by a driver and a turning vehicle wheel which turns according to a motion of the steering wheel do not have a mechanical link to each other. The steering apparatushas a steering wheelmanipulated by the driver. An angle of rotation of the steering wheelfrom a neutral position is a steering angle. The steering angle is detected by a steering angle sensor, and a turning actuatorturns a turning wheelbased on the detected steering angle. In a typical vehicle, the turning wheelis a front wheel. An angle of turn of the turning wheelfrom a neutral position is a turning angle. The turning angle is detected by a turning angle sensor. When the vehicle is traveling straight forward, the steering wheeland the turning wheelare at the neutral positions, and the steering angle and the turning angle are 0°. The steering wheeland the turning wheelare rotatable and turnable both clockwise and counterclockwise. The rotation of the steering wheeland the turning of the turning wheelare symmetric. Therefore, in the following, only the rotation and the turning in one direction will be described. Accordingly, in the following, a “large” steering angle and a “large” turning angle refers to the absolute values thereof.

The steering wheelis coupled to a steering shaft. A reaction force actuatoris connected to the steering shaftvia a connection mechanism. The connection mechanismconnects the reaction actuatorand the steering shaftin such a manner that a motion of the reaction force actuatorcorresponds to a rotation of the steering shaft. The reaction force actuatormay be formed from an electric motor. In this case, a steering angle of the reaction force actuator (electric motor) and a rotational angle correspond to each other. The connection mechanismmay be a gear mechanism, and in particular may be a gear pair formed from a worm and a worm wheel. The reaction force actuatorapplies to the steering wheela reaction force torque according to a vehicle velocity, the steering angle, a steering angle velocity, and the like. The vehicle velocity is detected by a vehicle velocity sensor. A steering torque sensorwhich detects a torque applied to the steering shaftdue to the steering by the driver is provided on the steering shaft. A steering torque T detected by the steering torque sensoris used for feedback control of the reaction force actuator. Through the application of the reaction force torque, the driver can feel the reaction of the steering manipulation. In addition, the steering angle sensordescribed above may be a sensor that detects a rotational angle of a rotor of the electric motor employed as the reaction force actuator.

The turning wheelis rotatably supported on a steering knuckle (not shown). A knuckle armof the steering knuckle is linked to a tie-rodin such a manner as to be bendable at a linkage point. The tie-rodis linked to a steering rackin such a manner as to be bendable at a linkage point. The steering rackhas a rack tooth, which engages a pinion. The pinionis fixed coaxially on a pinion shaft. The pinion shaftis connected to the turning actuatorvia a connection mechanism. The connection mechanism may be a gear mechanism, and in particular, a gear pair formed from a worm and a worm wheel. The connection mechanismconnects the turning actuatorand the pinion shaftin such a manner that a rotation of the pinion shaftcorresponds to a motion of the turning actuator. When the turning actuatoroperates, the pinionrotates, and the steering rackmoves along a longitudinal direction thereof. The movement of the steering rackis transferred to the turning wheelvia the tie-rodand the knuckle arm, effecting turning of the turning wheel. The turning actuatormay be formed from an electric motor. In this case, a rotational angle of a rotor of the electric motor and the turning angle correspond to each other. In addition, the turning angle sensordescribed above may be a sensor that detects a rotational angle of the rotor of the electric motor employed as the turning actuator. The turning actuatoris feedback-controlled based on the turning angle detected by the turning angle sensor.

The reaction force actuatorand the turning actuatorare controlled by a control apparatus. The control apparatuscalculates a target reaction force T* serving as a control target, based on a vehicle velocity v from the vehicle velocity sensor, a steering torque T from the steering torque sensor, and a steering angle θfrom the steering angle sensor. The control apparatuscontrols the reaction force actuatorso that the steering torque T becomes the target reaction force T*. In addition, the control apparatuscalculates a target turning angle θ* serving as a control target based on the steering angle θ. The control apparatuscontrol the turning actuatorso that the turning angle θbecomes the target turning angle θ*. The control apparatusmay be formed from one processing unit. Alternatively, the control apparatusmay be formed from a plurality of processing units which apply processes with functions of the control apparatusdistributed.

Because in the steer-by-wire steering apparatus the steering wheel and the turning wheel do not have a mechanical link, it can be difficult for the driver to understand that the turning angle has reached an end of a turning range. In the steering apparatus, a steering range of the steering wheelis virtually defined in correspondence to the turning range. This steering range will be hereinafter referred to as a virtual steering range. The control apparatuscontrols the reaction force actuatorso that the reaction force applied by the reaction force actuatorrapidly increases when the steering angle θbecomes close to an end of the virtual steering range. The reaction force actuatorapplies a large reaction force to the steering shaftat the end of the virtual steering range. The driver can understand that the turning angle has reached the end of the turning range because the reaction force to the steering manipulation of the steering wheelbecame large. This reaction force which increases at the end of the virtual steering range will be hereinafter referred to as an end steering reaction force. The end steering reaction force simulates the feeling of manipulation in the conventional steering apparatus.

In the steer-by-wire steering apparatus, a ratio of the turning angle θto the steering angle θis set according to the vehicle velocity, and, in particular, is set to a large ratio in comparison to the conventional steering apparatus in a vehicle velocity range with a low vehicle velocity. When the steering angle θof the steering wheelis the same, the turning wheelis turned larger in the steer-by-wire steering apparatus than in the conventional steering apparatus. In the case of rapid steering performed suddenly by the driver such as the time of emergency avoidance, the steering angle θto be steered by the driver does not significantly differ between the steer-by-wire steering apparatus and the conventional steering apparatus. Therefore, in the case of rapid steering, with the steer-by-wire steering apparatus there may be instances in which the steering angle θbecomes excessive.

Control during the rapid steering in the steering apparatuswill now be described.

is a diagram showing a response of the turning angle θwith respect to the steering manipulation during rapid steering. In, a solid line A shows a response in the steer-by-wire steering apparatus, and a broken line B shows a response in the conventional steering apparatus (non-steer-by-wire steering apparatus). In, a region R with hatching is a region in which a lateral force (cornering force) generated in the turning wheelis saturated or reduced during the rapid steering. This region will be hereinafter referred to as a lateral force reduction region R.

It is known that, during emergency avoidance, the driver will commonly suddenly rotate the steering wheelby a certain angle, regardless of whether or not the steering apparatus is the steer-by-wire steering apparatus. The turning angle θin this case is θin the conventional steering apparatus. However, in the steer-by-wire steering apparatus, the steering angle θreaches a limit θof the manipulation range (refer to) before the driver rotates the steering wheelby the certain angle described above. The turning angle θin this case is θ. The turning angle θis larger than the turning angle θduring the rapid steering in the conventional steering apparatus. As shown in, in the case of the steer-by-wire steering apparatus, the turning angle θis largely situated in the lateral force reduction region R. This shows that an increase of the lateral force corresponding to the increase in the turning angle θis not obtained. Therefore, much of the steering operation at this point is meaningless. In addition, because the steering angle θis also enlarged, when the driver returns the steering wheelafterwards, the return of the turning wheeltends to be delayed. In consideration of this situation, it is desirable that the turning angle θdoes not enter the lateral force reduction region R.

The steering apparatusaccording to the present embodiment narrows the virtual steering range during the rapid steering by generating the end steering reaction force at a smaller steering angle θ. With this configuration, excessive turning of the turning wheelis suppressed. The virtual steering range may be narrowed during the rapid steering so as to realize a turning angle similar to that in the conventional steering apparatus.

is an explanatory diagram of judgment of rapid steering such as the time of emergency avoidance. The horizontal axis of a graph shown inrepresents a turning angle velocity ω, and the vertical axis represents a turning follow delay Δθ(=θ*−θ) which is the delay of the actual turning angle θwith respect to the target turning angle θ*. Alternatively, the horizontal axis may represent a steering angle velocity ω. The turning angle velocity ωmay be calculated from the turning angle θby the control apparatus. Further, the steering angle velocity ωmay be calculated from the steering angle θby the control apparatus.

When the turning angle velocity ωis close to 0, the steering is not rapid steering. Therefore, when the turning angle velocity ωis smaller than or equal to a threshold ωthe control apparatusjudges that the steering state is not the rapid steering state. Further, in the steering apparatus, the turning follow delay Δθis also considered as a parameter for judging the rapid steering. When the driver is performing the counter-steer manipulation, a frictional force between the turning wheeland the road surface is reduced, and thus, the turning follow delay Δθis small. In addition, during the counter-steer manipulation, the steering angle θis large, and it is not desirable to narrow the virtual steering range. Therefore, when the turning follow delay Δθis small, the control apparatusjudges that the steering state is not the rapid steering state. There is a positive correlation between the turning angle velocity ωand the turning follow delay Δθ. As such, a boundary of the rapid steering judgement based on the turning follow delay Δθis a continuously-increasing straight line passing through the origin, as shown by a straight line C in.

When the turning wheelcontacts a curb, while the turning follow delay Δθis large, the turning angle velocity ωis small. In this case, a different control from that during the rapid steering is applied in order to apply a reaction force simulating the contact with the curb. Therefore, the threshold ωof the turning angle velocity ωmay be set so as to not interfere with the reaction force control during the contact with the curb.

Based on the above, the control apparatusjudges that the steering state is the rapid steering state when the turning angle velocity ωis greater than or equal to the threshold ω, and the turning follow delay Δθis greater than or equal to the boundary C which increases with the increase of the turning angle velocity ω. Hatched region Q inis the rapid steering region during the rapid steering state.

When the control apparatusjudges that the steering state is the rapid steering state, the control apparatussets the virtual steering range narrower in comparison to that at a normal time.shows an example of control of the end steering reaction force. The horizontal axis of a graph shown inrepresents the steering angle θ. The vertical axis represents the steering reaction force, that is, the steering torque T applied to the steering shaft. In, an upper limit value of the virtual steering range during the normal time is shown by a normal-time upper limit steering angle θ. During the normal time, when the steering angle θbecomes close to the normal-time upper limit steering angle θ, the reaction force actuatorapplies an end steering reaction force T, which rapidly increases, to the steering shaft, as shown by a broken line in. On the other hand, during the rapid steering, the control apparatussets a rapid-steering upper limit steering angle θwhich is smaller than the normal-time upper limit steering angle θ. The reaction force actuatorapplies an end steering reaction force Tduring the rapid steering to the steering shaftbased on the rapid-steering upper limit steering angle θ. The end steering reaction force Tduring the rapid steering is a reaction force obtained by shifting the end steering reaction force Tduring the normal time to the left in, and the shapes of the end steering reaction forces Tand Tare identical to each other.

is a block diagram showing generation of the end steering reaction force during the rapid steering. The control apparatusjudges the rapid steering based on whether or not the relationship among the target turning angle θ*, the actual turning angle θ, and the turning angle velocity ωor the steering angle velocity ωis within the rapid steering region Q of(S). When the control apparatusjudges the rapid steering, the control apparatussets up a rapid steering flag. Further, when the rapid steering flag is set, the control apparatussets a narrower virtual steering range than the normal time, and generates the end steering reaction force when the steering angle θis smaller than the normal time (S). The control apparatuscontrols the reaction force actuator, and the reaction force actuatorapplies the end steering reaction force corresponding to the narrower virtual steering range to the steering shaft.

As the turning angle velocity ωor the steering angle velocity ωbecomes larger, and/or as the turning follow delay Δθbecomes larger, the virtual steering range may be set to be narrowed stepwise or continuously.

Alternatively, the judgment of the rapid steering may be performed regardless of the turning follow delay Δθ, and based only on the turning angle velocity ωor the steering angle velocity ω.